1. Field of the Invention
The invention generally relates to methods and apparatus used in Fiber Distributed Data Interface (FDDI) networks to monitor link quality and isolate faults. More particularly, the invention relates to methods and apparatus which continuously monitor bit error rate at the physical (PHY) management layer of the FDDI hierarchy, using existing line status information, to localize and isolate faults quickly and accurately.
2. Description of the Related Art
FDDI token ring networks are well known to those skilled in optical data path communications technology. FDDI is a result of American National Standards Committee X3T9 and grew from the need for high speed interconnection among main frames, minicomputers and associated peripherals. It supports a variety of front-end, back-end and backbone networks configured in a variety of topologies and provides for secure 100 and 200 megabit per second transmission across long distance links (e.g., 100 km), with excellent immunity to the effects of electrical radiation and common mode voltages.
In order to appreciate the context in which the novel tester methods and apparatus are used, a brief description of the structure of an FDDI token ring network will first be set forth.
At least part of the rationale behind organizing FDDI as a ring is based on the nature of optical communication. Bus and passive star topologies would require the optical transmission to be detected at several sources simultaneously. Although fiber-optic taps are currently becoming available, the optical attenuation caused by such a device would severly restrict the number of nodes on the network.
Fiber-optic communication is still best suited for point-to-point transmission. Two types of Local Area Network (LAN) topologies can be realized with point-to-point links: the active hub star and the ring. Active stars introduce a single failure point that can disable the entire LAN. Single-ring networks also are prone to failures at any node. FDDI alleviates this problem with the dual-ring approach.
An FDDI ring typically comprises a variety of station types. Class A stations connect to both the primary and secondary rings of the network and are often referred to as "dual attachment stations". Data flows in opposite directions on the two rings. The Class A station can act as a wiring concentrator, serving to interconnect several single-attachment or Class B stations to the ring. Wiring concentrators give the network administrator a single maintenance point for a large number of stations. Class B attachments trade lower implementation costs and ease in servicing against the fault tolerance afforded in a Class A station.
The FDDI defined in X3T9 relates to the lower layers of the Open Systems Interconnection/International Organization for Standardization (OSI/ISO) model as follows.
The lowest layer of the OSI model, the Physical Layer, is described in two documents. The first, the FDDI Physical Medium Dependent (PMD) document, details optical specifications for FDDI. PMD defines the wavelength for optical transmission, the fiberoptic connector employed, and the function of the optical receiver. PMD also details an optional optical by-pass switch that can be incorporated within a station.
The second document describes the FDDI Physical Sublayer (PHY) which is the upper sublayer within the OSI Physical Layer. PHY defines the 4B/5B group-encoding scheme used to represent data and control symbols on the network. PHY also describes the method for retiming transmission within the mode.
The Data Link Layer in the OSI model is often subdivided into two sublayers: Link Layer Control (LLC) and Media Access Control (MAC). FDDI defines the lowest of these sublayers, MAC. Among other things, MAC defines the recovery mechanisms required for FDDI.
Another key element in the FDDI standards is Station Management (SMT). SMT falls outside of the scope of the OSI model and provides the intelligence that allows cohesive operation of the individual sublayers in an FDDI node. SMT defines error detection and fault isolation algorithms.
Having briefly described the structure and components of an FDDI ring it should be clear that physical link integrity needs to be assured. Thus, in high speed token ring networks, it is important to monitor the quality of the physical links on a continuous basis, identify bad links and isolate them. One means of identifying a bad link is to maintain a bit error rate count and exclude a link whenever a threshold of bit error rate, determined by the network manager, is exceeded. This prevents error propagation and insures that network throughput is efficiently maintained.
The FDDI standard has specified certain services in its MAC layer/SMT layer interface for the purpose of monitoring frames with errors or violation symbols. Though it serves as a measure of the quality of the physical connection between adjacent MACs, these services do not monitor an idle ring. Also, these services fail to isolate the fault to a specific physical link if there are many intervening physical links in between adjacent MACs. This situation is bound to happen in a secondary ring with fewer MACs if not all dual attachment stations have two MACs in them. In such cases, a need arises to monitor link quality at the PHY layer level.
Moreover, dual MACs cannot isolate single errors in single attachment stations connected to a concentrator. This is because the MAC at the concentrator sees only the final PHY which attaches to its single attachment station. Though known connection management (CMT) methods and apparatus can take care of long term noise, these methods and apparatus cannot account for single errors.
For the aforementioned reasons, it would be desirable to be able to continuously monitor link quality at the PHY layer without using the MAC.